1. Field of the Invention
The present invention relates to a sheet coil and a method for producing the sheet coil.
2. Description of the Prior Art
Heretofore, wire-winding methods have been used as a method for producing coils. However, the coils produced according to such methods have a low occupying rate of the conductor so that it is difficult to make coils small and thin, and further, precision of arrangement is not satisfactory. Therefore, improvement in accuracy of rotation of motors produced by using the coils is limited.
Methods for producing sheet coils using copper foil by means of photoetching are known. According to these methods, the resulting space between the conductors of a coil is two times or more the thickness of the conductor of the coil, and therefore there also exists a limitation for producing precise coils having a high occupying rate of conductor.
Other methods utilizing photoetching comprise etching, peeling the photosensitive resist and metalplating. The conductor gap is narrowed by the deposited metal and simultaneously the conductor becomes thick, and therefore, when the coil is laminated, the driving force of the sheet coil is lowered and, in addition, the resulting uneven thickness of the metal layer formed by plating increases the surface irregularity and thereby, after lamination, leakage often occurs disadvantageously between the adjacent sheets.
Further, there are known methods for producing sheet coils by a photoelectroforming method utilizing photoetching. A photosensitive resin pattern having recess portions in a form of spiral is formed on a base material composed of an insulating substrate and a metal layer overlying the insulating substrate, and then a metal plating layer is formed in the recess portions by plating. The photosensitive resin pattern is removed and then the metal layer on the insulating substrate corresponding to the portions which have been covered with the photosensitive resin pattern is removed by etching to produce sheet coils.
According to the above-mentioned methods. both the metal layer to be removed by etching and the metal plating layer are made of the same material so that the metal plating layer as well as the metal layer are removed to almost the same degree by the etching step. Therefore, it is difficult to maintain accuracy of the conductor thickness and conductor width; and therefore precise sheet coils can be produced with only a limited degree of accuracy.